42 Mo 95.95

Molybdenum

Transition metal Molybdaenum solid

[Kr] 4d⁵ 5s¹ · 2 · 8 · 18 · 13 · 1

← Nb Niobium Tc Technetium →

A refractory metal whose main job is to keep steel strong when it is hot. Molybdenum also sits at the centre of nitrogenase, the enzyme with which bacteria fix atmospheric nitrogen.

3D model

Bohr model: nucleus and electron shells from the real configuration. Valence electrons are highlighted.

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Atomic properties

Atomic number: 42
Atomic mass: 95.95 а.е.м.
Electron configuration: [Kr] 4d⁵ 5s¹
Electrons per shell: 2 · 8 · 18 · 13 · 1
Block: d-block
Group: Chromium group
Period: 5
Electronegativity (Pauling): 2.16
Electronegativity (Allen): 1.47
Atomic radius: 190 pm
Covalent radius: 154 pm
Van der Waals radius: 209 pm
Ionisation energy 1: 684.3 kJ/mol
Ionisation energy 2: 1 560 kJ/mol
Ionisation energy 3: 2 618 kJ/mol
Ionisation energy 4: 4 480 kJ/mol
Ionisation energy 5: 5 257 kJ/mol
Electron affinity: 71.9 kJ/mol
Common oxidation states: +4, +6
Oxidation states: -4, -2, -1, +1, +2, +3, +4, +5, +6

Physical properties

State at 25 °C: solid
Density: 10.28 g/cm³
Melting point: 2 896 K · 2 622.9 °C
Boiling point: 4 912 K · 4 638.9 °C
Speed of sound: 5 400 m/s

Thermal properties

Heat of fusion: 37.48 kJ/mol
Heat of vaporisation: 617 kJ/mol
Specific heat capacity: 0.251 J/(g·K)
Thermal conductivity: 138 W/(m·K)

Mechanical properties

Young's modulus: 329 GPa
Shear modulus: 126 GPa
Bulk modulus: 230 GPa
Poisson ratio: 0.31
Mohs hardness: 5.5
Brinell hardness: 1 500 MPa

Electrical and magnetic properties

Electrical resistivity: 53.4 nΩ·m
Magnetic ordering: paramagnetic
Curie point: no data
Néel point: no data
Superconducting point: 0.92 K

Crystal structure

Crystal structure: body-centred cubic (bcc)
Lattice constants: a = 314.7 pm

Abundance

In the crust: 1.2 mg/kg
In the ocean: 0.01 mg/L
In the universe: 0.5 mg/kg
In the human body: 0.07 mg/kg

Isotopes

Isotope	Abundance	Half-life	Decay mode
⁹²Mo	14.649 %	stable	—
⁹⁴Mo	9.187 %	stable	—
⁹⁵Mo	15.873 %	stable	—
⁹⁶Mo	16.673 %	stable	—
⁹⁷Mo	9.582 %	stable	—
⁹⁸Mo	24.292 %	stable	—
¹⁰⁰Mo	9.744 %	stable	—

Molybdenum-99, half-life 66 hours, is the most sought-after medical radioisotope in the world: it generates the technetium-99m used in roughly 30 million procedures a year.

Discovery

Year of discovery: 1778
Discovered by: Carl Wilhelm Scheele
Where: Sweden
Origin of the name: from Greek molybdos (lead): molybdenite was long confused with galena

History

Molybdenite was long confused with graphite and galena — hence the name, from the Greek for lead. Scheele showed in 1778 that it held a new element, and Hjelm isolated the metal in 1781.

Where it occurs

1.2 mg/kg of the crust. The main ore is molybdenite, MoS₂, mined both in its own right and as a copper by-product.

How it is produced

Roasting molybdenite to the oxide, then reducing with hydrogen. Much of it goes to steelmaking as ferromolybdenum.

Role in living things

An essential micronutrient: molybdenum sits in nitrogenase, sulfite oxidase and xanthine oxidase. Biological nitrogen fixation runs almost entirely on the molybdenum enzyme.

Safety

Of low toxicity to humans; in ruminants excess molybdenum induces secondary copper deficiency.

Uses

Alloying steel for heat resistance, corrosion resistance and hot strength
Molybdenum disulfide as a dry lubricant that works in vacuum and under extreme load
Electrodes and heating elements in high-temperature furnaces
Hydrotreating catalysts in oil refining
Molybdenum-99 as the parent isotope of technetium-99m in nuclear medicine

Curiosities

Molybdenite is visually indistinguishable from graphite — both soft, grey and greasy to the touch.
Without molybdenum there would be no biological nitrogen fixation, and so no protein on the planet.
Molybdenum disulfide lubricates where oil would evaporate — in vacuum and in space.
German heavy artillery in the First World War used molybdenum steel: it held up to barrel heating.